The invention relates to anticorrosive white pigments, consisting of TiO2 base bodies, preferably in the rutile modification and which have been surface modified, and to a method for producing the same, as well as to the use thereof.
Usually, TiO2 pigments having a particle size of 0.4 to 8.0 xcexcm and an average grain distribution d50 of 0.1 to 1.0 xcexcm are required in order to lighten dark media, or are used to give coating materials covering power. TiO2 pigments are all the more better suited for this the more the refraction of light of the pigments is superior to the refraction of light of the binding agent. The properties of coatings are substantially influenced by the incorporated TiO2 pigments. This behaviour in terms of application technique is adjusted during the production of the TiO2 pigments. This relates not only to the optical properties, such as scattering power, brightness and lustre, but also to further parameters, such as weather resistance, dispersability, settling behaviour and wettability.
The increasing importance of corrosion prevention and recognition of the health risks originating from known anticorrosive pigments which contain lead and chromate, has led to the development of less toxicologically harmful anticorrosive pigments based on phosphates, in particular Zn3(PO4)2, as well as molybdate pigments, tungstate pigments and zirconate pigments, metallic zinc, organic pigments and pure barrier pigments, such as micaceous iron ore. Anticorrosive pigments consisting of combinations of phosphates with borates or silicates are also known (Ruf, J.: Organischer Metallschutz, Vincentz Publishing House, Hannover 1993, in particular pages 248 to 289).
TiO2 pigments in the rutile modification are known from DE-A-2 951 805, the surface of the TiO2 pigments being coated with a first layer of cerium radicals and phosphate radicals and with a second layer of aluminium radicals and phosphate radicals to give improved optical properties. GB-B-2 115 394 describes TiO2 pigments which are surface-coated to improve the light stability and to make them suitable for use in laminates. TiO2 pigments having a corrosion-inhibiting action, the particles of which are sheathed with an outer layer of hydrated silicon dioxide, hydrated aluminium oxide or a mixture of these two components, are the subject matter of EP-B-0 412 686. Cations which effect corrosion prevention by means of ion exchange are chemically fixed to the coating, which cations consist of Ca ions, Zn ions, Li ions, Ba ions, Mg ions, Sr ions, Co ions or Pb ions. EP-A-0 595 341 refers to a coating material which contains TiO2 pigments which are coated with 1 to 10% by weight of a zinc alloy and are used for corrosion prevention. DE-C-2 200 656 deals, among other things, with TiO2 pigments with molybdate deposited on the particle surface thereof, which pigments are used as a corrosion-inhibiting admixture in coating materials. In accordance with US-A-4 017 315, such a coating can additionally contain metal phosphate. TiO2 pigments which are coated with lead chromate, strontium chromate or barium chromate or lead benzoate or calcium plumbate and are used as a corrosion-inhibiting admixture in coating materials are presented in GB-A-1 044 350. Additionally, a coating of one or more metal oxides and/or a phosphate can be applied. CZ-B-235 851 proposes, among others, TiO2 pigments which are coated with a layer of Mn3(PO4)2 and mixed with varnishes for the purpose of corrosion prevention.
The TiO2 pigments usually used in practice today as white pigments and fillers in varnishes and dyes have excellent optical properties in the incorporated medium because of their grain size of 0.1 to 1.0 xcexcm, which optical properties are characterised above all by light scatter.
This object is achieved by anticorrosive white pigments comprising surface-coated TiO2 base bodies, preferably in the rutile modification, characterised in that Mn3 (PO4)2 and one or more of the substances chosen from aluminum oxides, aluminum hydroxides, aluminum phosphates, aluminum hydrogen phosphates, aluminum dihydrogen phosphates and aluminum polyphosphates are precipitated on to the TiO2 base bodies.